Devices such as mobile phones, digital cameras, PDAs, or touch panel displays show a tendency of further prevalence.
Hitherto, an acrylic resin substrate has been used as cover glass for displays in those applications. However, because the acrylic resin substrate has a low Young's modulus, when a display was pressed, for example, with a finger, there was a case where the acrylic resin substrate bends and comes into contact with the display, thereby causing a display failure. Further, the acrylic resin substrate is easily scratched and therefore, its visibility is apt to degrade.
When a glass substrate is used as a protective member, the above problems can be solved. When the glass substrate is used as the protective member, the following properties are required for the glass substrate:
(1) the glass substrate must have high mechanical strength;
(2) the glass substrate must have small variations in strength;
(3) the glass substrate can be supplied in large quantities at a low cost; and
(4) the glass substrate must be excellent in bubble quality.
Heretofore, to meet the above requirement (1), a glass substrate tempered by ion exchange or the like (so called “tempered glass substrate”) is used (refer to Patent Document 1 and Non-patent Document 1).